Motors and GeneratorsJoe Khachan and Chris Stewart

IntroductionThis part of the syllabus draws upon many ideas and concepts from the old HSC syllabus,perhaps with a bit more of a historical twist thrown in. The concepts of the motor effect and thegenerator effect are taken beyond the basics into such realistic applications as three-phasemotors and generators, induction motors and so on.

This development of the basic ideas can seem a bit of a stretch, but you can always bring theideas back to just a few basic principles: the motor effect, its reverse the generator effect, andLenzs law. Or, if you prefer, you can relate all the ideas back to the concepts of the magneticforce on a moving charge and the magnetic field produced by a moving charge.

Some core physical conceptsWhile the physics involved with motors, generators and transformers can seem difficult andcomplex, when you dig down there are really just a few powerful ideas. You can always return tothe same concepts when discussing these subjects because, at the core, its all about magneticforces on electric charges.

1. A moving charge in a magnetic field experiences a forceA charge q moving with a speed v relative to a magnetic field B experiences a force F equal to

F = qvB sin ,where is the angle between the direction of the field and the direction of the charges velocity.The force is in a direction that is perpendicular to both the velocity and the field as given by theright-hand rule.

Why? There is no why. This is the way electromagnetism works. A negatively-charged particle(say, an electron) will experience a force in the opposite direction to a positively charged particle.Notice here we have said moving relative to the magnetic field. You can always look at theworld from the point of view of the charge in that frame of reference, the charge is stationaryand the magnetic field is changing around it. If the charge is moving, there is a force; if the fieldis changing, there is a force.

B field into page

Velocity

Force Path of positively-charged particle

2. A moving charge generates its own magnetic fieldIn particular for our purposes here, an electric current produces a magnetic field. The fieldencircles the current, as given by another right-hand rule.

Again, there is no point in asking why a current produces a field like this. It does. Electric charge,electric fields, magnetic poles and magnetic fields were all brought together in Maxwellsequations for the electromagnetic theory. Nature shows us these properties, and the equationsdescribe them that is all we know.

Amazingly, these two ideas can be used to describe everything about motors, generators andtransformers. Relating back to these two concepts throughout this section of the course will allowstudents to get used to the ideas, to practice them and apply them, and maybe even to see the

simplicity within complex pieces of machinery like a real generator or motor.

The motor effectConcept 2 above gives us the basis for the motor effect: if a current flows in a conductor, and theconductor sits in a magnetic field, then the conductor experiences a force on it in the directionyou would expect from the right-hand rule. If you have the equipment, you can demonstrate themotor effect on a beam of charge by using a cathode ray tube in which the electron beam isvisible and a permanent magnet. The magnetic field bends the electron beam youre using acurrent and a magnetic field to produce a change in motion.

You can do many simple things with the motor effect before you get even close to a motor. Twocoils of wire hanging parallel side by side, each carrying a current, will attract or repel dependingon whether the currents are in the same or opposite directions.

One way of looking atthe field around a

current-carrying wire

Current out of the page,field lines going anti-

clockwise

The field as dots andcrosses

Current sourceForce

You can also make the worlds simplest loudspeaker all you need is a signal generator (or aradio with an amplifier), a good strong magnet (neodymium types work well) and a fine, light coilof wire (I find very light gauge wire wound around a light plastic spool works well). Connect thecoil to the signal generator set at an audible frequency (somewhere in the hundreds of hertz).Place the magnet in the middle of the coil on the bench, and place a plastic cup or yoghurt tub or

something similar on top. You should be able to hear sound coming from your speaker.

Youre hearing the sound because the current in the coil creates a magnetic field. The magnetsown field interacts with the coils such that the coil is either pushed up or pulled down, dependingon the direction of the current in the coil. The up and down moves the bottom of the plastic cup this is your speaker cone, pushing the air back and forth making sound waves.

The generator effectThe reverse of the motor effect, the generator effect is a result of concept 1 above: a changingmagnetic field near a conductor (either through relative motion of a magnet and the conductor orthrough an increasing or decreasing field strength) produces an electric current in the conductor.You can always trace this back to the force on a charge from a magnetic field.

One point associated with these concepts is Lenzs law that the induced current is always in adirection such that its magnetic field opposes the changing field that created it. This, too, can betraced back to the two concepts listed at the start of this paper, but can perhaps be more easilygrasped as an application of the principle of conservation of energy. If a changing field produceda current, and that current produced a field that assisted the changing field, you would producemore current, changing the field more, producing more current quickly you would have acurrent increasing towards infinity and an out-of-control magnetic field. You would get energyfrom nothing.

So if the magnetic field is changing one way, the current induced will always oppose this change if the field is increasing, say, the current will produce a field to oppose this increase. Movingthe North pole of a magnet towards a coil will produce a current in the coil that reduces theoverall magnetic field.

The simplest demonstrations involve a coil, a magnet and a galvanometer. You can use the right-hand rule to work out which way the current is moving in the coil to satisfy Lenzs law.

AC Signal generatoror radio

Magnet

Coil

Plastic cup

More interesting is the issue of eddy currents because you can do some really nicedemonstrations. Get a length of copper pipe (or some other good conductor that is not also easilymagnetised) and a really good, strong magnet. Hold the pipe vertically and drop the magnet downthe pipe. As the magnet falls, taking its magnetic field with it, eddy currents are induced in thepipe. These produce fields that oppose the field that created them: which the current in aparticular part of the pipe pushes back on the magnet as it approaches from above and pulls up onthe magnet as it falls away.

With a really strong magnet you have time to watch it fall. For a twist, if you have the tools, youcan cut a thin gap down the length of the tube so that the eddy currents do not have a completecircuit around the pipe. This reduces the amount of current that can flow in the pipe though itcan still flow in small circles within the wall of the pipe.

MotorsApplying the motor effect to make a real motor involves some thought. The first step is notingthat circular motion is much easier to sustain than linear motion (this is perhaps worth discussingwith students it may or may not be obvious to them, or to you, why a linear motor isnt easy tobuild).

The DC motor is the easiest to build: all you need is a battery, a magnet, some insulated wire (thetype where the wire is galvanised and the coating can be stripped off easily, not the kind sheathedin plastic) and some supporting scaffolding.

N

Magnet moving to the left

Field near coil pointing to the left, increasing

Current flows so thatfield produced decreases

field from magnet

NFalling magnet

Eddy current

Eddy current

Copper pipe

Cross-section ofcopper pipe with gap eddy currents flowin the walls but notaround the pipe.

A DC motor needs a commutator of some kind because you need to swap the direction of thecurrent in the coil twice each rotation. This simple motor has a primitive version of a commutator the insulation is stripped around only half of one end so that the current only flows during halfof each turn:

Thats a simple motor. For more complex motors, you may have a motor kit at your school thatincorporates split-ring commutators, multiple coils etc. If you dont, we recommend searching forsome old commercial motors you can rip apart. Motors from washing machines, starter motorsfrom car engines, motors from toys or household appliances the older the better, since modernmotors are often sealed inside moulded plastic casings that make them hard to get at.

Pull the motor apart and identify the bits. Is it AC or DC? An induction motor? Single or multi-phase? Being able to identify the different parts and figure out how the motor works demystifiesthe object and reinforces the ideas.

If you can get your hands on a generator a dynamo from bike, an old fuel generator or similar you could try pulling that apart to see what you can find. You could try using a motor as agenerator, though only some kinds will work this way. (Motors that dont have permanentmagnets wont work as generators because there is no magnetic field to induce a current unless acurrent is already flowing a bit of a circular argument there. Worth seeing if the students canwork this out.)

A few web resources The UniServe Science site for the Motors and Generators module a whole pile of

information and links: http://science.uniserve.edu.au/school/curric/stage6/phys/motors/ The UNSW HSC web site: http://www.phys.unsw.edu.au/%7Ejw/HSCmotors.html

Hold magnet above(or fix to base)

Battery

Wire stands

Coil made from enamelled copper wire stripone end completely, strip only half of the otherend so that current only flows for half a turn

One particularly good link: Magnet Man Rick Hoadley, all sorts of experiments andexplanations: http://www.execpc.com/~rhoadley/magindex.htm

Interactive Java tutorials on electromagnetism, including magnetic fields, induction,generators, Lenzs law, speakers and transformers:http://micro.magnet.fsu.edu/electromag/java/index.html

The HyperPhysics site at Georgia State University good set of brief, succinctexplanations that are all cross-referenced: http://hyperphysics.phy-astr.gsu.edu/hbase/magnetic/elemot.html